Google will not disclose the cost of its self-driving car project, or any of its other far-out research and development efforts, a spokesperson told us. It has always characterized self-driving cars as a small side bet.

And it's true: Even if Google is spending millions of dollars—at least $3.6 million just on its current fleet of cars, never mind the engineering salaries—that's a drop in the bucket compared to its overall revenues.

But if Google intends to take this technology to the mainstream, as seems to be its ambition, it must reduce the cost of its sensors.

Use cheaper sensors. The startup founder we spoke to said that once Google got its data-processing algorithms right, it could rely on less-sensitive sensors and use sheer computing power to make up for their lower accuracy.